diff --git a/frontend/src/app/services/theme.service.spec.ts b/frontend/src/app/services/theme.service.spec.ts
index d4c8915..56b6b07 100644
--- a/frontend/src/app/services/theme.service.spec.ts
+++ b/frontend/src/app/services/theme.service.spec.ts
@@ -3,9 +3,26 @@ import { vi } from 'vitest';
 import { ThemeService } from './theme.service';
 
 describe('ThemeService', () => {
+  beforeEach(() => localStorage.clear());
+
+  it('defaults to the operating-system preference', () => {
+    vi.stubGlobal(
+      'matchMedia',
+      vi.fn(() => ({ matches: true, addEventListener: vi.fn() })),
+    );
+    const service = TestBed.inject(ThemeService);
+    TestBed.tick();
+
+    expect(service.preference()).toBe('system');
+    expect(document.documentElement.dataset['theme']).toBe('dark');
+  });
+
   it('persists explicit and system preferences', () => {
     const listener = vi.fn();
-    vi.stubGlobal('matchMedia', vi.fn(() => ({ matches: true, addEventListener: listener })));
+    vi.stubGlobal(
+      'matchMedia',
+      vi.fn(() => ({ matches: true, addEventListener: listener })),
+    );
     localStorage.setItem('lmnp-theme', 'system');
     const service = TestBed.inject(ThemeService);
     TestBed.tick();
diff --git a/frontend/src/app/services/theme.service.ts b/frontend/src/app/services/theme.service.ts
index fa2ef6f..3674ff7 100644
--- a/frontend/src/app/services/theme.service.ts
+++ b/frontend/src/app/services/theme.service.ts
@@ -40,6 +40,6 @@ export class ThemeService {
     const storedPreference = localStorage.getItem(THEME_STORAGE_KEY);
     return storedPreference === 'dark' || storedPreference === 'system'
       ? storedPreference
-      : 'light';
+      : 'system';
   }
 }
diff --git a/frontend/src/index.html b/frontend/src/index.html
index 00ff979..2cec810 100644
--- a/frontend/src/index.html
+++ b/frontend/src/index.html
@@ -1,16 +1,28 @@
 <!doctype html>
 <html lang="en">
+  <head>
+    <meta charset="utf-8" />
+    <title>LmnpFrontend</title>
+    <base href="/" />
+    <meta name="viewport" content="width=device-width, initial-scale=1" />
+    <script>
+      try {
+        const preference = localStorage.getItem('lmnp-theme');
+        const theme =
+          preference === 'light' || preference === 'dark'
+            ? preference
+            : matchMedia('(prefers-color-scheme: dark)').matches
+              ? 'dark'
+              : 'light';
+        document.documentElement.dataset.theme = theme;
+        document.documentElement.dataset.themePreference =
+          preference === 'light' || preference === 'dark' ? preference : 'system';
+      } catch {}
+    </script>
+    <link rel="icon" type="image/x-icon" href="favicon.ico" />
+  </head>
 
-<head>
-  <meta charset="utf-8">
-  <title>LmnpFrontend</title>
-  <base href="/">
-  <meta name="viewport" content="width=device-width, initial-scale=1">
-  <link rel="icon" type="image/x-icon" href="favicon.ico">
-</head>
-
-<body>
-  <app-root></app-root>
-</body>
-
-</html>
\ No newline at end of file
+  <body>
+    <app-root></app-root>
+  </body>
+</html>